Water-in-oil emulsions are dispersions of discontinuous or discrete aqueous particles commonly referred to as the "internal" aqueous phase in a continuous or "external" oil phase. Emulsions can contain as much as and more than 70 volume percent internal phase. These are often referred to as high internal phase emulsions (HIPEs). The volume fraction of the internal aqueous phase in such emulsions can be as high as 90 percent and frequently is as high as 95 percent with some HIPEs being reported as high as 98 percent aqueous phase.
The use of high internal phase emulsions (HIPEs) in forming porous polymeric materials is well known. See for example: U.S. Pat. Nos. 5,210,104; 5,200,433; 4,536,521; 4,788,225; 5,147,345; 5,331,015; 5,260,345; 5,268,224 and 5,318,554. HIPE polymerization has gained increasing interest as open-celled polymeric foams having the capacity to absorb relatively high amounts of water and other liquids can be produced. In the described HIPEs, the external oil phase typically comprises a vinyl polymerizable monomer, such as 2-ethylhexyl acrylate and styrene, and a cross-linking monomer such as divinylbenzene. The internal aqueous phase typically comprises water, a radical initiator (if not in the oil phase) and an electrolyte. To form a stable emulsion, a HIPE surfactant is added to the oil phase prior to emulsification. Commonly used HIPE stabilizing surfactants include nonionic surfactants such as sorbitan esters (for example, sorbitan monooleate and sorbitan monolaurate). Other known HIPE stabilizing surfactants include certain polyglycerol aliphatic esters such as those described in U.S. Pat. No. 5,500,451 and 5,817,704, the entire contents of which are incorporated herein by reference.
In order to effectively stabilize HIPEs, the aforementioned surfactants must be used in relatively high concentrations (typically above 5 weight percent of the oil phase). When used at lower concentrations (e.g., less than 5 weight percent based on the weight of the oil phase, or about 0.06 weight percent based on the weight of the entire emulsion), these surfactants do not effectively stabilize a HIPE through polymerization to an open-celled foam. See J. M. Williams, D. A. Robleski, Langmuir, 4, (1988) 656-662.
Not only does the use of a surfactant add significantly to the cost of HIPE materials, in many applications residual surfactant must be removed from the final product, thus further adding to the cost of producing HIPE materials. For example, as described in U.S. Pat. No. 4,788,225, the surfactant may be an extractable residue which can be removed through post-polymerization rinses. If not removed, the surfactant residue may create a problem when it comes in contact with sensitive human skin.
Therefore, it is desirable to prepare open-celled polymerized HIPE foams with as little extractable surfactant as possible. One way to reduce the amount of extractable surfactant is to use a polymerizable surfactant, as described in U.S. Pat. No. 5,977,194 the contents of which are incorporated herein by reference. Another way is to include a long hydrophobic component which is sufficient to mechanically bind the surfactant into the foam during polymerization.
It would be desirable to provide surfactants which can effectively stabilize HIPEs and form open-celled HIPE materials at relatively low concentrations (e.g., below about 2 weight percent based upon the total weight of the oil phase of the emulsion) and/or which produce HIPE materials with relatively small amounts of extractable surfactants.